Night observations of free-ranging Rhesus monkeys

The nocturnal distribution and behavior of individually marked Macaca mulatta were studied at the La Parguera, Puerto Rico, colony of the Caribbean Primate Research Center. The new image intensifier was used successfully to identify 399 monkeys in 185 sleeping clusters. Monkeys moved into mangrove trees close to favorite feeding areas usually 35 minutes after sunset. The group condensed to less than one-half the daytime spread, vocalizations increased and grooming ceased. Movements and vocalizations ceased several hours after sunset, although bursts of activity occurred throughout the night. Activity resumed 40 minutes before sunrise. Activity was higher during full moon, when I observed feeding, play and sexual behavior. Fights at night were twice as frequent during the breeding season. Monkeys slept in clusters of one to four, 58% of which were of two. Sixty-three percent were composed of maternal relatives, 33% were mother-infant pairs. Mature males clustered with non-related males, slept alone or with females (in the breeding season). Yearlings slept with their mothers or with older siblings. Distribution of monkeys in a group at night reflects daytime associations.